1. Field of the Invention
The present invention relates to receivers for detecting and recovering data from received spread spectrum signals.
2. Description of the Prior Art
Multiple access radio communications techniques provide a facility for supporting several radio communications channels contemporaneously within the same radio frequency bandwidth. One such multiple access technique is Code Division Multiple Access (CDMA). Those acquainted with the field of radio communications will appreciate that the term CDMA is applied to several known radio access interfaces such as IS-95, Wide band CDMA, Joint Detection also known as TD-CDMA as proposed for third generation mobile radio, as well as other fixed and mobile radio standards. The term CDMA is therefore not limited to any particular radio access standard but should be interpreted as including any radio access interface for providing contemporaneous communications to a plurality of users using spread spectrum radio communications.
Generally, with a CDMA communication system the data to be communicated is arranged to modulate a spreading code. The modulated spreading code has a bandwidth which is much wider than the bandwidth of the base band signal. At a receiver the spreading code is reproduced and correlated with the received spread-spectrum signal. A so called “coding gain” is produced as a result of the correlation, between the reproduced spreading code, and the modulated spreading code present in the received spread spectrum signals. Data symbols can be recovered from the result of the correlation in the presence of other spread spectrum signals produced from different spreading codes.
Known receivers for detecting and recovering data from spread spectrum signals include the Rake receiver. The Rake receiver is provided with a plurality of correlators, each of which is arranged to correlate the received signal with a reproduced version of the spreading code offset in time by a different amount. Each correlator thereby generates an output which is combined to form a composite signal from which the data can be detected. In this way the data can be recovered in the presence of multi-path propagation, by setting the respective delays in correspondence with an anticipated spread of delays of each of the paths of the multi-path propagation profile.